Native lignin is a naturally occurring amorphous complex cross-linked organic macromolecule that comprises an integral component of all plant biomass. The chemical structure of lignin is irregular in the sense that different structural units (e.g., phenylpropane units) are not linked to each other in any systematic order. It is known that native lignin comprises pluralities of two monolignol monomers that are methoxylated to various degrees (trans-coniferyl alcohol and trans-sinapyl alcohol) and a third non-methoxylated monolignol (trans-p-coumaryl alcohol). Various combinations of these monolignols comprise three building blocks of phenylpropanoid structures i.e. guaiacyl monolignol, syringyl monolignol and p-hydroxyphenyl monolignol, respectively, that are polymerized via specific linkages to form the native lignin macromolecule.
Extracting native lignin from lignocellulosic biomass during pulping generally results in lignin fragmentation into numerous mixtures of irregular components. Furthermore, the lignin fragments may react with any chemicals employed in the pulping process. Consequently, the generated lignin fractions can be referred to as lignin derivatives and/or technical lignins. As it is difficult to elucidate and characterize such complex mixture of molecules, lignin derivatives are usually described in terms of the lignocellulosic plant material used, and the methods by which they are generated and recovered from lignocellulosic plant material, i.e. hardwood lignins, softwood lignins, and annual fibre lignins.
Native lignins are partially depolymerized during the pulping processes into lignin fragments which are soluble in the pulping liquors and subsequently separated from the cellulosic pulps. Post-pulping liquors containing lignin and polysaccharide fragments, and extractives, are commonly referred to as “black liquors” or “spent liquors”, depending on the pulping process. Such liquors are generally considered a by-product, and it is common practice to combust them to recover some energy value in addition to recovering the cooking chemicals. However, it is also possible to precipitate and/or recover lignin derivatives from these liquors. Each type of pulping process used to separate cellulosic pulps from other lignocellulosic components produces lignin derivatives that are very different in their physico-chemical, biochemical, and structural properties.
Given that lignin derivatives are available from renewable biomass sources there is an interest in using these derivatives in certain industrial processes. For example, U.S. Pat. No. 5,173,527 proposes using lignin-cellulosic materials in phenol-formaldehyde resins. A. Gregorova et al. propose using lignin in blends or alloys with polypropylene for its radical scavenging properties (A. Gregorova et al, Radical scavenging capacity of lignin and its effect on processing stabilization of virgin and recycled polypropylene, Journal of Applied Polymer Science 106-3 (2007) pp. 1626-1631). However, large-scale commercial application of the extracted lignin derivatives, particularly those isolated in traditional pulping processes employed in the manufacture of pulp and paper, has been limited due to, for example, the inconsistency of their chemical and functional properties. This inconsistency may, for example, be due to changes in feedstock supplies and the particular extraction/generation/recovery conditions. These issues are further complicated by the complexity of the molecular structures of lignin derivatives produced by the various extraction methods and the difficulty in performing reliable routine analyses of the structural conformity and integrity of recovered lignin derivatives.
Formaldehyde-based resins such as phenol formaldehyde (PF), urea formaldehyde and melamine formaldehyde are extremely common and used for a variety of purposes such as manufacturing of housing and furniture panels such as medium density fiberboard (MDF), oriented strand board (OSB), plywood, and particleboard. Concerns about the toxicity of formaldehyde have led regulatory authorities to mandate the reduction of formaldehyde emissions (e.g. California Environmental Protection Agency Airborne Toxic Control Measure (ATCM) to Reduce Formaldehyde Emissions from Composite Wood Products, Apr. 26th, 2007).
The addition of phenol-free and formaldehyde-free compositions, such as sulphur-free lignin derivatives, represents a viable alternative to the toxic chemicals. However, past attempts at adding lignin derivatives, such as Alcell® lignin, to PF resins have been largely unsuccessful due to the relatively poor performance characteristics of the final product. For example, the normalized Alcell® lignin-PF resin bond strength at 150° C. was 3,079 MPa*cm2/g as tested by the ABES method (Wescott, J. M., Birkeland, M. J., Traska, A. E., New Method for Rapid Testing of Bond Strength for Wood Adhesives, Heartland Resource Technologies Waunakee, Wis., U.S.A. and Frihart, C. R. and Dally, B. N., USDA Forest Service, Forest Products Laboratory, Madison, Wis., U.S.A., Proceedings 30th Annual Meeting of The Adhesion Society, Inc., Feb. 18-21, 2007, Tampa Bay, Fla., USA). These values are significantly lower than what is commonly expected for instance for plywood or OSB PF resins (3,200-3,600 MPa*cm2/g).